Quadrupole ion traps are used in mass spectrometers to trap ions, i.e., atoms or molecules having a charge due to the loss or gain of one or more electrons. Quadrupole ion traps use electromagnetic fields generated by applying RF signals between elongate rods (or poles) to trap ions radially within a defined volume of space that will be referred in this disclosure to as a trapping volume. Quadrupole ion traps additionally use end caps axially offset from one another to trap the ions axially within the trapping volume.
Ion traps can be used for many different purposes in mass spectrometry and other fields. For instance, they can be used to store ions temporarily while the ions are waiting to be transferred to another part of a scientific instrument, such as a measurement stage. Likewise, they can be used to temporarily store of ions after the ions are created or after the ions exit a measurement stage of a scientific instrument.
Quadrupole ion traps also are often used for separating certain ions from other ions based on the mass to charge ratio (m/z) of the ions. Specifically, the electromagnetic fields that trap ions in the ion trap can be manipulated so that ions having an m/z ratio above or below a certain m/z ratio are ejected from the trap, while other ions having different m/z ratios remain in the trap.
It also is known to use an ion trap as a fragmentation cell in which ions are fragmented into smaller pieces. In an example, an inert gas such as argon is introduced into the trapping volume. The ions trapped in the trapping volume collide with the molecules of the inert gas with sufficient force to fragment the ions. The fragments and remaining intact ions are then ejected from the trap (either selectively based on m/z ratio or in their entirety) for further processing. For instance, the fragments and ions may be ejected toward a detector for measurement. Alternatively, in a tandem mass analyzer, the fragments and ions may be ejected into another mass analyzer stage, e.g., a Fourier transform mass analyzer, RF quadrupole mass analyzer, time of flight mass analyzer, or another quadrupole ion trap mass analyzer.
As mentioned above, quadrupole ion traps use electromagnetic fields to contain the ions within the trapping volume both radially and axially. Ions can be admitted or ejected from the ion trap by altering the electric fields (e.g., turning one or more of the electric fields off or changing the amplitude and/or frequency of one or more of the electric fields) so that the ions, or at least ions having certain m/z ratios, enter or exit the trapping volume. In most quadrupole ion traps, ions enter the trapping volume travelling axially through one of the ends of the trapping volume. Many quadrupole ion traps also permit ions to exit the trap travelling axially through one of the ends of the trapping volume, typically, the end axially opposite from the entrance end. However, the ions may enter and exit the trapping volume through the same end. Other ion traps eject ions radially. Specifically, a gap may be provided in one or more of the elongate poles through which ions can exit travelling radially.
Generally, the ions are contained radially within the trapping volume by an RF containment field generated by applying an RF signal to the poles. Typically, the RF signal is a differential signal, and the in-phase component of the RF signal is applied to two opposing poles of the quadrupole and the antiphase component of the RF signal is applied to the other two opposing poles of the quadrupole.
With respect to axial containment, a quadrupole ion trap that traps ions of only a single polarity at any given instant typically axially contains the ions by applying a DC voltage to each of the axial end caps. This potential causes the ions to travel back and forth in the axial direction within the trapping volume.
However, a DC field cannot trap both positive and negative ions simultaneously because a particular axial DC field will provide an effective barrier for ions of one polarity, but would accelerate the ions of the opposite polarity axially out of the trapping volume.
U.S. Pat. No. 7,227,130 discloses a technique for generating axial RF fields that can simultaneously contain ions of both positive and negative polarities both axially and radially. Specifically, this patent discloses the application of particular RF signals between the quadrupole rods to generate a radial RF containment field in conjunction with the application of other RF signals between the end caps and the rods to generate an axial RF containment field between the end caps and the rods of the quadrupole. The axial RF containment field keeps ions of both polarities trapped and circulating between the two end caps.
One drawback of the technique described in the U.S. Pat. No. 7,227,130 is that the axial containment field and radial containment field are interdependent, i.e., they interact with each other. Consequently, one cannot be changed without affecting the other. Thus, for instance, changing the radial containment field to reduce the trapping volume radially would also change the axial containment field. To restore the axial containment field to its original state would require that the RF signals applied to the end caps be adjusted accordingly.